Liquid Organic Hydrogen Carriers (LOHCs): Toward a Hydrogen-free

Article pubs.acs.org/accounts

Liquid Organic Hydrogen Carriers (LOHCs): Toward a Hydrogen-free Hydrogen Economy Patrick Preuster,† Christian Papp,‡ and Peter Wasserscheid*,†,§ †

Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Egerlandstrasse 3, 91058 Erlangen, Germany ‡ Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Egerlandstrasse 3, 91058 Erlangen, Germany § Forschungszentrum Jülich GmbH, Helmholtz-Institut Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstrasse 3, 91058 Erlangen, Germany CONSPECTUS: The need to drastically reduce CO2 emissions will lead to the transformation of our current, carbon-based energy system to a more sustainable, renewable-based one. In this process, hydrogen will gain increasing importance as secondary energy vector. Energy storage requirements on the TWh scale (to bridge extended times of low wind and sun harvest) and global logistics of renewable energy equivalents will create additional driving forces toward a future hydrogen economy. However, the nature of hydrogen requires dedicated infrastructures, and this has prevented so far the introduction of elemental hydrogen into the energy sector to a large extent. Recent scientific and technological progress in handling hydrogen in chemically bound form as liquid organic hydrogen carrier (LOHC) supports the technological vision that a future hydrogen economy may work without handling large amounts of elemental hydrogen. LOHC systems are composed of pairs of hydrogen-lean and hydrogen-rich organic compounds that store hydrogen by repeated catalytic hydrogenation and dehydrogenation cycles. While hydrogen handling in the form of LOHCs allows for using the existing infrastructure for fuels, it also builds on the existing public confidence in dealing with liquid energy carriers. In contrast to hydrogen storage by hydrogenation of gases, such as CO2 or N2, hydrogen release from LOHC systems produces pure hydrogen after condensation of the high-boiling carrier compounds. This Account highlights the current state-of-the-art in hydrogen storage using LOHC systems. It first introduces fundamental aspects of a future hydrogen economy and derives therefrom requirements for suitable LOHC compounds. Molecular structures that have been successfully applied in the literature are presented, and their property profiles are discussed. Fundamental and applied aspects of the involved hydrogenation and dehydrogenation catalysis are discussed, characteristic differences for the catalytic conversion of pure hydrocarbon and nitrogen-containing LOHC compounds are derived from the literature, and attractive future research directions are highlighted. Finally, applications of the LOHC technology are presented. This part covers stationary energy storage (on-grid and off-grid), hydrogen logistics, and on-board hydrogen production for mobile applications. Technology readiness of these fields is very different. For stationary energy storage systems, the feasibility of the LOHC technology has been recently proven in commercial demonstrators, and cost aspects will decide on their further commercial success. For other highly attractive options, such as, hydrogen delivery to hydrogen filling stations or direct-LOHC-fuel cell applications, significant efforts in fundamental and applied research are still needed and, hopefully, encouraged by this Account. the only byproduct.3 If the applied hydrogen is produced from renewable energy (e.g., by water electrolysis4) or without CO2 byproduct creation (e.g., by methane decomposition to carbon and hydrogen5), a CO2-free energy system is possible. (2) Hydrogen is characterized by a very high gravimetric energy density. Its lower heating value is 33.3 kWh/kg or 120 MJ/kg− no other energy carrier shows a higher gravimetric energy density (see Figure 1).

1. INTRODUCTION The concept of using hydrogen as an energy carrier is not new. As early as 1976, Jones postulated the use of hydrogen as energy carrier not only to be desirable but inevitable on the long-term.1 Since that time, numerous studies highlighting various aspects of using hydrogen as an energy vector have been published. These were summarized by Bockris in 2013 in form of a historic record of scientific developments toward a future hydrogen economy.2 The use of hydrogen as energy vector has many advantages: (1) Combustion of hydrogen in fuel cells or in combustion chambers typically liberates water as © 2016 American Chemical Society

Received: September 20, 2016 Published: December 22, 2016 74

DOI: 10.1021/acs.accounts.6b00474 Acc. Chem. Res. 2017, 50, 74−85

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Accounts of Chemical Research

Figure 1. Energy density of various energy carriers in comparison: left side, volumetric energy density; right side, gravimetric storage density.

gravimetric energy density.12 As indicated by the title of this contribution, we will not consider in the following state-of-theart technologies to store and transport hydrogen in its elemental form as either compressed hydrogen (cH2, 200− 700 bar)13 or cryogenic hydrogen (l-H2, −253 °C).14 Moreover, we will restrict our discussion to technologies that offer the potential for storage and transport of hydrogen on the TWh scale and thus may potentially act as backbone of a future hydrogen economy. For this reason, we will also not treat hydrogen storage in the form of physisorption or chemisorption to solids.15 This contribution deals with the scientific foundations and recent progress of technologies that aim for chemical hydrogen storage by binding hydrogen to hydrogen-lean molecules in catalytic hydrogenation reactions. To act as a hydrogen storage system, the so-formed hydrogen-rich molecules should enable, at least in principle, release of the stored hydrogen in a catalytic dehydrogenation to close the storage cycle. Two classes of hydrogen-lean molecules are distinguished: (1) Hydrogen-lean molecules that are extracted from the atmosphere or exhaust gas mixtures, like CO2 or N2. They form, after catalytic hydrogenation, the well-known bulk chemicals formic acid,16 methane,17 methanol,18 Fischer−Tropsch products19 (depending on the applied catalyst and process parameters during the hydrogenation process) or ammonia,20 respectively. (2) Hydrogen-lean organic liquids that enable fully reversible catalytic hydrogenation/dehydrogenation cycles. For these compounds the term liquid organic hydrogen carriers (LOHCs) has been coined (in an earlier publication also the term “liquid organic carriers” was used21). Note that LOHC systems enable hydrogen storage without binding or releasing other substances from or to the atmosphere. Pure H2 is obtained from LOHC dehydrogenation after appropriate condensation of the liquid, ideally high-boiling carrier molecule. The hydrogen-rich LOHC compounds can be stored for extended times without energy losses and transported over long distances using established energy transport logistics for liquid fuels (e.g., pipelines, ships, trucks). Thus, transition to a hydrogen economy using existing and depreciated infrastructure appears feasible. In the following, we will limit our contribution to hydrogen storage and handling using LOHC systems. While earlier reviews on the topic have given a broad overview of liquid organic and inorganic hydrides, their specific properties, and related catalyst systems for

However, while the chemical industry produces and consumes more than 60 million tons of hydrogen every year,6 the introduction of hydrogen as energy vector is struggling, mainly for two reasons: (1) Hydrogen has to be produced from other forms of energy. This is different from our previous understanding of energy carriers. Moreover, hydrogen production from coal, oil, and gas (e.g., by steam reforming or partial oxidation)7 liberates CO2 as byproduct. For example, 1 ton of hydrogen produced by methane reforming has been reported to liberate more than 10 tons of CO2.8 In renewable energy scenarios, hydrogen is expected to be produced by electrolysis of water. However, this method of hydrogen production is, at today’s gas and electricity prices, still more expensive than hydrogen production from natural gas. While production costs for 1 kg of hydrogen from electrolysis have been reported to be around 3 € kg−1,9 the same amount produced from methane is about half this cost, depending on the local gas price. (2) It is complicated and costly to store and transport large amounts of elemental hydrogen. As shown in Figure 1, the excellent gravimetric energy storage density of H2 comes with an extremely low volumetric energy density under normal conditions of only 0.01 MJ/L or 3 Wh/L. This is a direct consequence of the low density of hydrogen under ambient conditions (0.0898 g/L at 0 °C and 1 bar).10 Moreover, hydrogen is easily combustible with air in very broad concentration ranges (Table 1). It is characterized by low Table 1. Relevant Properties of Hydrogen for a Safe and Secure Handling10,11 ignition limits in air (%) ignition energy in air (mJ) flame temperature (°C) boiling point (K) diffusion coefficient in air (cm2 s−1)

4−75 0.02 2045 20 0.610

ignition energy and high diffusion rates in gases, liquids, and even solids. At room temperature, hydrogen shows a negative Joule−Thomson coefficient thus heating up when expanded. Consequently, the handling of elemental hydrogen requires investment into specialized equipment and dedicated infrastructures. Numerous hydrogen storage technologies have been investigated in the recent decades. They all target to increase the volumetric energy content without compromising on 75


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Scheme 1. Schematic View on Hydrogen Storage Using the Dibenzyltoluene (H0-DBT)/Perhydrodibenzyltoluene (H18-DBT) LOHC System

Scheme 2. Schematic Representation of the Most Relevant Reaction Steps in Hydrogen Storage Using the N-Ethylcarbazole (H0-NEC)/Perhydro-N-ethylcarbazole (H12-NEC) LOHC System

hydrogen release,22 our contribution focuses on a narrow set of most promising LOHC systems and covers the full range from fundamental catalytic aspects to first demonstrator applications.

hydrogen from the product stream. Another problematic feature of the TOL/MCH LOHC system is its low flashpoint that is below the temperature of dehydrogenation. Despite these practical disadvantages, the Japanese company Chiyoda Corporation has developed the TOL/MCH LOHC system into a demonstrated technology and has recently announced its large-scale application for hydrogen and energy logistics.27 Attempts to overcome some of the specific problems of the TOL/MCH hydrogen storage pair while sticking to pure hydrocarbon systems for full compatibility with the fuel infrastructure led later to a closer investigation of the naphthalene/tetralin/decalin,28 as well as the benzyltoluene (H0-BT)/perhydrobenzyltoluene (H12-BT) and dibenzyltoluene (H0-DBT)/perhydro-dibenzyltoluene (H18-DBT), storage systems (hydrogen capacity = 6.2 mass %).29 While naphthalene is solid at room temperature (mp = 80 °C), the LOHC systems using technical mixtures of benzyltoluene or dibenzyltoluene regioisomers turned out very attractive (Scheme 1). They combine very wide liquid ranges (mp = −34 °C; bp = 390 °C in case of H0-DBT) with excellent thermal robustness, low flammabilities and very favorable toxicological profiles. Most importantly, these mixtures are already in large-scale technical use as heat transfer oils (e.g., under the trade name “Marlotherm”) for many decades, which guarantees industrial acceptance, full registration, technical availability at low price, and well established quality standards in their production.

2. AROMATIC HYDROCARBONS AS HYDROGEN-LEAN STORAGE MOLECULES Historically, the first research activities toward hydrogen storage using LOHC systems date back to the 1980s. The first oil crises promoted research at the Paul Scherrer Institute in Switzerland toward the use of nuclear power for mobility via water electrolysis, hydrogen storage, and on-board fuel cells. Already at these early times, the advantages of LOHC systems vs battery technologies (e.g., higher energy storage capacities, shorter refueling times) were recognized.23 All these early studies focused on the LOHC system toluene (TOL)/ methylcyclohexane (MCH). This system has a hydrogen storage capacity of 6.1 mass % hydrogen (1.55 kWh/L) and shows a comparatively high heat of hydrogenation of −68.3 kJ/ mol H2. For thermodynamic reasons, this leads to harsh conditions in dehydrogenation if full conversion to TOL is desired (typical conditions for 99% MCH conversion are 320 °C at 1 bar H2).24 At such harsh conditions, the formation of side products by dealkylation or coking was typically observed with the applied heterogeneous Pt25 or Ni catalyst systems.26 At both hydrogenation and dehydrogenation conditions, all reactants of the TOL/MCH storage system are gaseous. Due to the relatively low boiling point of all components, extensive condensation and purification steps are required to isolate pure 76


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containing LOHC systems due to a limited world production of boron would be an issue, as pointed out by Crabtree.40 An alternative, very interesting concept has been recently presented by Milstein et al.41,42 It leaves the traditional track of hydrogen storage by hydrogenation of aromatic compounds. The authors report instead a LOHC-system based on 2aminoethanol that is catalytically converted into the cyclic peptide glycine anhydride while liberating 4 mol of hydrogen per mol of cyclic dipeptide formed (hydrogen capacity = 6.6 mass %).43 The reaction conditions are mild (105−135 °C) with the homogeneous Ru-complex (preferably carrying PNNH ligands) applied. The same complex is also able to catalyze the reverse hydrogenation reaction (10−70 bar H2 pressure, 110 °C). However, none of the reported storage cycles was 100% selective or complete even after prolonged reaction times (12−48 h for each individual step). Still, this new approach is highly remarkable and opens new avenues for future developments.

However, all pure hydrocarbon LOHC systems suffer from their relatively high heat of hydrogenation. In the case of the H0-DBT/H18-DBT, the latter has been determined experimentally to be −65 kJ/mol H2.30 As a consequence, catalytic dehydrogenation has to take place at temperatures above 250 °C. This high temperature level makes the utilization of “waste heat” to drive the endothermic dehydrogenation reaction more difficult.

3. N-HETEROAROMATICS AS HYDROGEN-LEAN STORAGE COMPOUNDS The development of LOHC systems with significantly lower heat of hydrogenation/dehydrogenation has been pioneered by researchers of the company Air Products and Chemicals who carried out a systematic theoretical screening of LOHC candidates according to this criterion.31 The best studied system identified in this screening is N-ethylcarbazole (H0NEC)/dodecahydro-N-ethylcarbazole (H12-NEC) (hydrogen capacity = 5.8 mass %) (Scheme 2). The dehydrogenation enthalpy of this system is 50 kJ/mol H2. To tackle one drawback of H0-NEC, namely, its melting point of 68 °C, Stark et al. have investigated eutectic mixtures of different N-alkyl substituted carbazoles and found melting points of optimized H0-N-alkyl carbazole mixtures down to 24 °C.32 Another drawback of N-alkyl carbazoles and their hydrogenated counterparts is the tendency to split off their N-alkyl substituent at temperatures far below the decomposition temperature of the heteroaromatic ring structure.33 LOHC systems with a dehydrogenation enthalpy similar to N-ethylcarbazole are indole/indoline (ΔHdehyd = 52 kJ/mol H2)34 and 1,2,3,4-tetrahydroquinoline/quinolone.35 These systems have been studied using homogeneous and heterogeneous catalysts, and dehydrogenation temperatures were found to be remarkably low. For indoline dehydrogenation, Jessop et al. found 81% dehydrogenation to indole using a Pd on silica catalyst at only 100 °C (reaction time 1 h).34 In the case of the dehydrogenation of 1,2,3,4-tetrahydroquinoline, the applied homogeneous Ir-complex was able to reach 100% dehydrogenation over 20 h at 138 °C (reflux of the solvent p-xylene).35 The use of other N-based heteroaromatic/heteroalicyclic LOHC systems (e.g., pyridines/piperidines or perhydronaphthyridines/naphthyridines) has been very recently summarized in a review by He et al.36 In brief, the presence of nitrogen in LOHC molecules always improves dehydrogenation thermodynamics and kinetics. However, it also promotes thermal lability and opens reaction pathways to undesired degradation products.

5. LOHC HYDROGENATION/DEHYDROGENATION CATALYSIS Catalysis plays a key role in hydrogen storage using LOHC systems as both H0-LOHC hydrogenation for hydrogen storage and Hx-LOHC dehydrogenation for hydrogen release require promotion by suitable catalyst systems. Despite the fact that a number of studies have applied homogeneous catalysts,21,35,42,44−47 the practical need for decoupling storage tank and reactor volume to independently adjust storage capacity (in MWh) and release power (in MW) calls for heterogeneous catalyst systems in large-scale storage applications. Therefore, we will restrict the following part to the development of heterogeneous catalysts for LOHC hydrogenation and dehydrogenation reactions. 5.1. Fundamental Aspects

While many applied studies have started with screening commercial catalysts for LOHC transformations,27 efficient catalyst optimization requires solid fundamental understanding. To this end, the groups of Steinrück and Libuda have investigated since 2011 the catalytic dehydrogenation of various hydrogen-rich LOHC molecules on single crystalline surfaces and supported model catalysts using a rigorous surface science approaches, i.e. applying spectroscopic methods under ultrahigh vacuum (UHV) conditions.48 By combining synchrotron radiation-based high resolution X-ray photoelectron spectroscopy (HR-XPS), infrared reflection absorption spectroscopy (IRAS), temperature-programmed desorption (TPD), molecular beam experiments, and theoretical studies, these groups were able to gain an unprecedented atomic/molecular-level understanding of the relevant processes in LOHC dehydrogenation catalysis. Some of the most instructive findings are summarized in the following: • In situ XPS during H12-NEC adsorption on Pt(111)33 and on Pd(111),49 followed by applying a heating ramp revealed a reaction sequence that starts with the desorption of H12-NEC multilayers followed by dehydrogenation to H8-NEC through formation of a central pyrrole unit. The latter dehydrogenates further to H0-NEC. At even higher temperatures, a C−N scission first forms carbazole before the LOHC molecule disintegrates on the hot surface at even higher temperatures (Scheme 2).

4. ALTERNATIVE LOHC SYSTEMS Even lower down is the hydrogenation/dehydrogenation enthalpy for boron containing LOHC systems. Müller et al. give 36 kJ/mol H2 for the dehydrogenation of 1,2-dihydro-1,2azaborine.37 Hydrogen release from molecular liquids containing nitrogen and boron has attracted significant attention since the 1960s.38 While H2 release has been reported at temperatures below 100 °C, many systems are solids and stability at storage conditions of up to 60 °C and in contact with moisture is a concern.39 Moreover, the reversibility of these systems is a grand challenge. To date, no such system has been reported that can be recharged with molecular hydrogen. If all these technical issues could be solved, still scalability of boron 77


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Scheme 3. Dehydrogenation of Dicyclohexylmethane (DCHM) on Pt(111) under the Conditions of a Surface Science Experiment as Determined by a Combination of HR-XPS, TPD, NEXAFS, and IRAS

• Dehydrogenation catalysis of several perhydro-N-alkylcarbazoles (with the alkyl groups being ethyl, propyl, or butyl), compared on Pt(111) and on Al2O3-supported Pt nanoparticles, showed no effect of the substituents on the order of reaction steps and their onset temperatures.50 • The undesired ethyl abstraction from H0-NEC to form high melting carbazole (mp = 245 °C) has been found to be a function of the Pt particle size. The structure sensitivity of dealkylation was studied by HR-XPS on Al2O3-supported Pt model catalysts and Pt(111) single crystals. On smaller, defect-rich Pt particles, the onset of dealkylation is shifted by 100 °C to lower temperatures compared to large, well-shaped particles and a wellordered Pt(111) surface.51 • The dehydrogenation of dicyclohexylmethane (DCMH) on a Pt(111) surface includes formation of a π-allylic species coadsorbed with hydrogen (−70 to −10 °C), followed by phenyl group formation and hydrogen desorption at −10 to 60 °C, and finally, full dehydrogenation (above 60 °C) accompanied by C−H bond scission at the methylene bridge (Scheme 3). The latter reaction is a first unwanted decomposition step. Further heating under the conditions of the surface science experiment led to fragmentation of the diphenylmethane molecules on the surface already at 180 °C.52 It is noteworthy that onset temperatures of catalytic dehydrogenation or decomposition are always found much lower in the surface science experiments compared to real conditions with the solid catalyst suspended in the LOHC medium. For example, the very low onset temperatures reported for dicyclohexylmethane under surface science conditions (see above) is contrasted by the fact that no dehydrogenation activity is observed for such molecule with any Pt catalyst system at 1 bar hydrogen atmosphere below 200 °C. Furthermore, decomposition of pure hydrocarbon LOHC compounds observed under surface science conditions at 180 °C is typically not observed under real conditions below 350 °C to the same extent. Along the same lines, it has been found that the optimum temperature for H12-NEC dehydrogenation at Pt(111) under surface science conditions is 110 °C,33 while optimum rates were found at 250 °C for the same reaction under real catalysis conditions.53 In order to close this “temperature gap”, H12-NEC, otherwise volatile under UHV was covalently linked to an imidazolium cation, forming a nonvolatile ionic liquid.54 In this way, it was possible to probe in situ the dehydrogenation of the “immobilized” and liquid H12-NEC close to equilibrium reaction conditions of real heterogeneous catalysis by XPS and TPD. Interestingly, dehydrogenation was observed under these conditions at identical reaction temperatures to the real catalysis experiment. This demonstrates that the availability of free and reactive coordination sites in proximity to the adsorbed LOHC

molecules plays a crucial role for the high reactivity detected in surface science experiments. 5.2. Applied Aspects of LOHC Hydrogenation

Hydrogenation of pure hydrocarbon aromatics with traditional heterogeneous catalysts is performed in the chemical industry on a million-ton-per-year scale, for example, benzene hydrogenation to cyclohexane for adipic acid production.55 In a similar manner the hydrogenation of H0-DBT is successfully carried out using commercial Ru on AlOx catalyst (50 bar, 150 °C),29 and recent mechanistic studies have even elucidated the hydrogenation order of the three aromatic rings applying 1H NMR spectroscopy.56 The hydrogenation of carbazole derivatives is also known in the literature but of significantly less technical relevance so far. Already in the 1940s, Adkins et al. have described the hydrogenation of different carbazole and indole derivatives using high temperatures and pressures (e. g., 200 bar H2, 260 °C).57 Following the seminal study by Pez et al. at Air Products in 2006,31 a number of academic research groups have significantly optimized the protocol for H0-NEC hydrogenation. Table 2 gives an overview of applied conditions and catalyst systems. Table 2. Overview of Various Catalyst Systems and Reaction Conditions for the Hydrogenation of N-Ethylcarbazole (H0NEC) as Described in Selected Publications active metal

support

Ni C/SiO2 Raney Ru

Pt

AlOx C TiO2/SiO2 C

temperature (°C)

pressure (bar)

refs

130−180 130 120−230 130 120−180 130 130 130

50−70 70 50 70 60−70 70 70 70

58 59 60 61 62−64 62 59 59

In brief, Ni, Ru, and Pt catalysts have been found suitable for the hydrogenation of H0-NEC. Typical applied temperatures are between 120 and 230 °C, and hydrogen pressures are between 50 and 70 bar. Interestingly, the work of Ye et al.58 revealed indications for the mass transport of hydrogen being the rate-determining step of the reaction. This is in contrast to the finding by Wan et al., who described that the variation of stirrer speed under very similar conditions had no effect on the observed reaction rate.63 The Ru-based hydrogenation catalyst was found to form H8-NEC as kinetically stable intermediate of the H0-NEC hydrogenation reaction.62 5.3. Applied Aspects of LOHC Dehydrogenation

Also the dehydrogenation of cyclic aliphatic hydrocarbons is well-known in the chemical and petrochemical industry. The most important example is the so-called “platforming”, a refinery process that converts alkanes into alkylated aromatics 78


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dehydrogenation productivities of up to 10.9 g(H2) g(Pt)−1 min−1, which corresponds to a quite impressive Pt-based power output of 22 kW g(Pt)−1.74 This performance was obtained using a homemade egg-shell catalyst. H12-NEC dehydrogenation was found to be heavily mass transfer influenced at temperatures above 235 °C. In full agreement with the surface science studies described above, the dehydrogenation of H12-NEC proceeds in a sequence of dehydrogenation and decomposition steps depending on the applied conditions. Yang et al. described, for example, that the dehydrogenation of H12-NEC to H8-NEC starts at Pd on AlOx already at 128 °C, while the dehydrogenation of H8-NEC to H4-NEC requires a minimum temperature of 145 °C, and complete dehydrogenation to H0NEC a temperature of 178 °C.72 The same study also demonstrates that under the applied mild conditions only 0.003 mol % ethane (from the hydrodealkylation of the N-ethyl group) is detected in the product mixture. Moreover, the obtained product gas stream was claimed to be free of PEM fuel cell poisons (PEM = proton exchange membrane). However, even under these mild conditions, 2% of the NEC carrier molecule was dealkylated to carbazole over 10 hydrogenation/ dehydrogenation cycles. The same group has also studied systematically the activity of different metals in H12-NEC dehydrogenation under comparable conditions and found the reactivity order of Pd > Pt > Ru > Rh.73

to boost the octane number of fuels. As indicated by its name, the process uses Pt-containing catalysts to promote dehydrogenation.65 Palladium and other metals have been also described but their activity is generally significantly lower.66 MCH dehydrogenation is both an important reaction in the platforming process and the key step in the hydrogen-release using the TOL/MCH LOHC system. Therefore, a number of studies have tried to improve lifetime, activity, and selectivity of MCH dehydrogenation catalysts. Small additions of rhenium have been found to enhance the stability of the catalyst.65 Moreover, doping with sulfur has shown beneficial effects.67 Furthermore, it has been shown that the active catalyst under real conditions is not pure Pt but a carbon modified Pt−C surface.68 Depending on the H2‑pressure, a balance between formation and degradation of the carbon layer is established. At hydrogen pressures below 1 bar, the catalyst deactivates much faster due to cooking. Okada et al. have demonstrated that a homemade Pt/AlOx dehydrogenation catalyst worked with stable catalytic performance (LHSV = 2 h−1, T = 320 °C) with 99.9% toluene selectivity in MCH dehydrogenation for 3000 h time on-stream.25 Also Ni catalysts have been found suitable for MCH dehydrogenation. However, these catalysts are less active compared to Pt and more prone to side product formation.69 Al-ShaikhAli et al. have very recently published a bimetallic NiZn catalyst that reaches one-third of the MCH conversion of a standard Pt catalyst (at 350 °C) with 90% selectivity to toluene at high conversion.70 This is a quite remarkable result compared to earlier reported Ni-based systems. As described above, hydrogen storage in the benzyltoluene (H0-BT)/perhydrobenzyltoluene (H12-BT) and dibenzyltoluene (H0-DBT)/perhydrodibenzyltoluene (H18-DBT) LOHC systems has significant advantages over the TOL/ MCH system, such as higher volumetric storage density, easier hydrogen purification, and reduced toxicity. Brückner et al. have studied the catalytic dehydrogenation of H12-BT and H18DBT using various commercial catalysts (310 °C, 1 bar H2) and found that Pt on C and Pt on AlOx catalysts are by far more active than the respective Pd analogues on the same supports.29 Different from dehydrogenation of pure hydrocarbon LOHC systems, the dehydrogenation of nitrogen-containing alicyclic compounds proceeds at lower temperatures due to their lower reaction enthalpy. Here, we will restrict ourselves to the specific case of H12-NEC dehydrogenation, as this is by far the bestinvestigated compound. Table 3 gives an overview of catalyst systems and reaction conditions that have been successfully applied in the literature for this purpose. In brief, H12-NEC dehydrogenation has been studied at 1 bar and at temperatures between 130 and 270 °C using various active metals. At 270 °C, Peters et al. demonstrated

6. APPLICATIONS AND OUTLOOK Applications of the LOHC technology with potential to contribute to a hydrogen-free hydrogen economy in the future can be envisaged in the fields of energy storage, hydrogen logistics, and hydrogen mobility. For stationary energy storage, it is useful to discriminate between grid and off-grid applications. In grid applications, the storage unit is always connected to an energy supply or sink of almost infinite dimension. The unit’s owner aims to fill the storage with cheap energy at energy-rich times, while the unit generates valuable energy in the form of electricity and heat at energy-lean times. This concept is attractive for buildings, business parks, or industrial sites if the energy price spread is large enough, the yearly hours of energy surplus/shortage are sufficiently high and the investment costs for the LOHC unit are reasonable.75 Off-grid applications are economically more attractive than on-grid applications as the electricity and heat produced by the system does not compete at grid price level but with other (usually more expensive) ways of off-grid energy production, for example, a diesel generator. Examples for such off-grid applications include tourist locations on secluded islands, transmitting stations in the desert, or mining exploration sites in the wilderness. Typical costs for electricity from diesel generators in such locations are 0.6 €/kWh, which allows to cover both the cost for renewable energy production and the LOHC storage unit. In all attractive off-grid application scenarios, the storage unit is embedded into an environment with excess supply in renewable energy. However, temporal energy demand does not fit the generation profile and anticipated energy-lean times are too long to bridge the resulting shortages with commercial battery systems. Figure 2 illustrates an example of off-grid power supply: Here, enough sunlight is available in daytime to fill the LOHC hydrogen storage and a heat storage system for overnight hydrogen release from the LOHC system.

Table 3. Overview of Various Catalyst Systems and Reaction Conditions for the Dehydrogenation of Perhydro-Nethylcarbazole as Described in Selected Publications active metal Ir Pd

Pt Ru Rh

support

temperature (°C)

pressure (bar)

refs

homogeneous SiO2 AlOx C AlOx AlOx AlOx

150 150−170 130−180 170 180−270 180 180

1 1 1 1 1 1 1

44 71 72 73 29, 72, 74 72 72 79


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Figure 2. LOHC-based unit for off-grid power supply: at energy-rich times, solar energy is used to operate photovoltaics and to charge both the LOHC and the heat storage system.

Figure 3. LOHC demonstration unit at Fraunhofer IAO, Stuttgart: 100 kW dehydrogenation unit (horizontal vessel in the middle) heated by a hydrogen burner (not visible behind the container) producing hydrogen for a 30 kW fuel cell (right in front). The total storage capacity of the unit is 1000 L of H18-DBT (tanks on the left) or 2.05 MWh based on the lower heating value (LHV) of the stored hydrogen.

Energy and hydrogen logistics will play a key-role in an increasingly renewable world as global regions with the highest potential for renewable energy harvest are not identical to those with the highest energy demand. A detailed evaluation of LOHC-based energy and hydrogen transport has been recently published based on two specific scenarios, namely, export of renewable energy from Northern Africa (solar) and export from Iceland (hydroelectric, geothermal, wind) to Germany.78 The study concludes that energy transport via LOHC over long distances is indeed quite promising. Compared to the transport of compressed or liquid hydrogen, transport costs for LOHCbased hydrogen are low, in particular due to the possibility to use existing fleets of product tankers. Compared to electric transmission, the fragmented energy delivery by dozens of individual ships navigating in the open sea increases security of supply. The additional storage function of the LOHC systems allows for energy release at the time and at the location of

A typical LOHC system for power storage involves the following five elements: electrolysis, LOHC hydrogenation unit, storage tanks, LOHC dehydrogenation unit, and fuel cell.76 A recent important progress in stationary energy storage is the so-called “OneReactor” concept77 that applies a combined hydrogenation/dehydrogenation unit. This concepts saves significant cost as only one reactor with its catalyst inventory, piping, manifolds, and management system is required. A great advantage of performing storage and release in the same reactor is that the reactor is always at elevated temperature. This greatly increases dynamics of the system as heating up times of cold reactors are avoided. A requirement to operate successfully the “OneReactor” is to have a catalyst system or combination of catalysts at hand that works for both hydrogenation and dehydrogenation with suitable activity and excellent selectivity. To identify and further optimize such catalyst systems is a very attractive research target. 80


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Figure 4. Process chain for operating a LOHC-based hydrogen filling station.

Figure 5. Schematic view of a direct LOHC fuel cell.

delivery of hydrogen to various customers. Note that LOHCbased hydrogen storage and transport is not restricted to green hydrogen from water electrolysis but can also be based on hydrogen from methane reforming or “waste” hydrogen from chlor-alkali electrolysis processes. LOHC-based hydrogen delivery to hydrogen filling stations is a realistic technology target for the upcoming three to five years. Major development targets deal with providing the required hydrogen quality for fuel cell vehicles at low investment into the LOHC dehydrogenation/compression/ purification unit. Options to provide the heat of dehydrogenation at a hydrogen filling station include direct electrical heating, burning of fuels (e.g., LPG or bioethanol), or heat storage systems that are electrically heated from solar or wind power at energy-rich times. Figure 4 shows the sequence of unit operations that is required to operate such a hydrogen filling station. As development goal for a future commercialization, it is envisaged to operate the whole process chain in a 20 feet container at the filling station (including the dispenser) and to use either the existing fuel tanks of the station or a small tank within the container for LOHC storage. On a longer research perspective, also on-board hydrogen generation on mobile platforms is a very attractive research and development goal. Technical realization of this vision is easier on heavy and large platforms, like ships or locomotives, than on high performing small cars. On-board storage of the

highest energy demand, a fact that adds significantly to the economics of the transport path. Figure 3 shows a very recently installed commercial demonstrator for LOHC-based hydrogen logistics. This demonstrator has been manufactured by Hydrogenious Technologies GmbH, Erlangen,79 and installed at Fraunhofer IAO, Stuttgart. The unit runs on H18-DBT that is delivered to Stuttgart from Erlangen via road transport. The material is charged in Erlangen using solar power, a PEM electrolyzer (Siemens), and a LOHC hydrogenation unit (Hydrogenious). The unit provides 30 kW power and is used as part of a microsmart grid80 to charge electric vehicles during the night at Fraunhofer IAO, Stuttgart. Note that from an actual economic viewpoint, it is not favorable to convert hydrogen, stored and transported in LOHC systems, back into electricity. In contrast, it is much more attractive to use the stored hydrogen directly as fuel for hydrogen mobility or in industrial applications. While 1 kg of hydrogen (33.3 kWh LHV) provides 18.3 kWh electricity (assuming a FC efficiency of 55%) resulting in a market value of 2.2 € (assuming the German industrial electricity prize of 0.12 €/kWh), the market value of 1 kg of hydrogen at a hydrogen filling station is between 9 and 12 €. Commercial hydrogen delivery costs are between 5 and 55 €/kg depending on purchase quantity, quality, and location. This price spread gives an indication of the economic value of on-demand 81


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density = 14−15 mW cm−2). In contrast to direct methanol fuel cells,85 the problem of undesired crossover of the fuel through the membrane is reduced. D-LOHC-FC concepts operating on N-containing LOHC systems were later published by Driscoll et al.86 (using indoline and N-benzylaniline as fuels) and Ferell et al.87 (using mainly H12-NEC and perhydrofluorene as fuels). A thermodynamic evaluation of several different LOHC fuels for direct fuel cell concepts has been presented by Arauja et al.88 Also the patent literature covers D-LOHC-FC concepts.89 However, significant problems are still encountered on the way to making D-LOHC-FC fuel cells efficient and robust. These include compatibility issues between the membrane and the LOHC materials, problems of electropolymerization, and generally low power outputs due to the slow kinetics. It is so far unpredictable whether the power output of D-LOHC-FCs will ever be sufficient to enable their reasonable use on mobile platforms. Therefore, the theoretical possibility of operating DLOHC fuel cells on vehicles one day should not prevent anybody from establishing 700 bar hydrogen filling stations today for the next decade of hydrogen mobility. Having said this, it is also true that the topic of D-LOHC-FC is fascinating with many interesting challenges for fundamental research. In conclusion, it appears from the current state of knowledge that LOHC systems offer significant potential to become a leading concept among the different options of chemical energy storage. Different from storage options for elemental hydrogen, the concept enables storage and transport of very large amounts of energy in existing infrastructures. Different from other “power-to-X” concepts, no additional reactants have to be isolated from gas mixtures or the atmosphere and very pure hydrogen can be provided from LOHC storage systems. The scientific and technological progress of the last five years toward robust LOHC systems and efficient hydrogen storage/hydrogen release processes is very encouraging and supports the vision of a future hydrogen economy in which much of the applied hydrogen may be bound to high boiling liquids. This would not only facilitate a stepwise transition into the hydrogen economy, it would also make this economy look similar to our current energy system. This is a very relevant point to gain public acceptance for this kind of breakthrough toward a more sustainable energy system.

diesel-like LOHC is straightforward, a double chamber tank system is required to accommodate Hx-LOHC and H0-LOHC in the same tank volume. During the refueling process, HxLOHC is filled into the one chamber of the tank system, while H0-LOHC is sucked off to empty the other tank chamber from uncharged LOHC.81 For obvious reasons, the provision of the dehydrogenation heat for hydrogen release is more challenging on mobile platforms than for stationary applications. Some options, like heat supply from a heat storage system, are impractical for mobile applications. Basically, the following options remain: • Electrical heating using the electricity produced by the on-board FC is an obvious option, but it comes with the shortest driving range. The reason is that the limited efficiency of the FC enters twice the overall efficiency calculation, for driving and for producing the electricity equivalent for heating the endothermic dehydrogenation. • On-board hydrogen combustion is the second option to produce the dehydrogenation enthalpy. At least 27% of the released hydrogen has to be burned for full hydrogen release in the case of H18-DBT (assuming no heat losses). Comparison with Figure 3 illustrates that significant progress in power density of the conversion units is needed to make this technology an option for onboard energy delivery on road vehicles. • Combining the endothermic LOHC dehydrogenation reaction with metal hydride hydrolysis as an exothermic hydrogen release process has been proposed by Jessop et al.82 While the idea appears appealing on the first view, handling of solid metal hydrides/oxides, controlled heat release from these systems, and their on-board regeneration is very challenging. • Heat integration between the exothermic fuel cell operation and the LOHC dehydrogenation is another option. As only PEM fuel cells fulfill the dynamic requirements for mobile applications, this requires on the first view LOHC dehydrogenation at the temperature level of PEM fuel cells (up to 180 °C).83 An alternative, very interesting but still very immature option is the onboard operation of a Direct-LOHC-fuel cell (D-LOHCFC, Figure 5). Conceptually, this option has the striking advantage that LOHC dehydrogenation and fuel cell operation takes place in one device and at one catalyst thus saving both space and investment. In addition, the D-LOHC-FC does not liberate elemental hydrogen from the LOHC but only protons, a process that can take place at the low temperatures of typical PEM fuel cells. Assuming that a D-LOHC-FC could reach one day the same hydrogen efficiency as today’s hydrogen cars (0.7 kg H2 consumption per 100 km driving range), a car with an 80 kg LOHC tank content (carrying a LOHC system with 62 g of releasable H2/kg LOHC) would offer a maximum driving range of ca. 700 km. Note that the D-LOHC-FC process handles no elemental hydrogen. Liquid, hydrogen-charged LOHC is pumped into the device, and electric power is delivered together with uncharged LOHC. Without doubt, efficient D-LOHC fuel cells would bring the vision of hydrogen-free, but still hydrogen-based energy converters much closer. But is this feasible? Already in 2003, Kariya et al. reported on the successful operation of a DLOHC-FC based on cyclohexane.84 The authors claim good performance (open circuit voltage = 920 mV, maximum power

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Peter Wasserscheid: 0000-0003-0413-9539 Notes

The authors declare the following competing financial interest(s): Peter Wasserscheid is co-founder of the company Hydrogenious Technologies GmbH (www.hydrogenious.net) that is mentioned in the Account. Biographies Patrick Preuster is group leader at the Institute of Chemical Reaction Engineering of the Friedrich-Alexander-University Erlangen-Nuremberg (FAU) (www.crt.cbi.fau.de) for energy conversion technologies using LOHC systems. His particular research focus is on catalytic conversion units with high power density. Christian Papp is group leader of the “surface and in situ spectroscopy group” at the Chair of Physical Chemistry II at the Friedrich82


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Alexander-University Erlangen-Nuremberg (FAU). His current research focuses on the fundamental understanding of surface processes on the atomic and molecular level. Peter Wasserscheid heads the Institute of Chemical Reaction Engineering of the Friedrich-Alexander-University Erlangen-Nuremberg (FAU) since 2003. In addition, he acts as founding director of the Helmholtz-Institute Erlangen-Nuremberg for Renewable Energies (www.hi-ern.de) and as director of the IEK-11 at the Forschungszentrum Jülich (www.fz-juelich.de) since 2013. He is cofounder of the company Hydrogenious Technologies GmbH (www.hydrogenious. net) that is mentioned in the article.

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ACKNOWLEDGMENTS The authors thank Prof. Hans-Peter Steinrück, Prof. Jörg Libuda, Prof. Wolfgang Arlt, Prof. Eberhard Schlücker, Dr. Andreas Bösmann, Dr. Daniel Teichmann, Dr. Caspar Paetz, Dr. Berthold Melcher, and Dr. Martin Schneider for fruitful discussions and helpful advice. Moreover, the authors thank the Energie Campus Nürnberg, the Bavarian Hydrogen Center and the German Science Foundation through its Cluster of Excellence “Engineering of Advanced Materials (EXC 315)” for financial support. C.P. wants to thank H.Z.B. for the allocation of synchrotron beamtime.

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Liquid Organic Hydrogen Carriers (LOHCs): Toward a Hydrogen-free

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